Savvy Scheduler
Scheduling
- Must efficiently select which process must be run
- For the sake of simplicity: Only considering single process/single thread single core systems
i.e. one thing running at a time
Key Terms - timestamps
- Arrival Time
- Start Time
- End Time
Key Terms - durations
- Response time = start time - arrival time
- Turnaround time = end time - arrival time
- Wait time = turnaround time - run time
Measures of efficiency
-
Lowest average turnaround time
-
Lowest wait time
-
Latency
Process State
- R - Running or Runnable
- S - Interruptible Sleeping (waiting on an event)
- D - Uninterruptable Sleep (IO typically, still on the CPU)
- T - Stopped
- Z - Zombie process (terminated, but not reaped)
Ready Queue
A queue of runnable processes, not waiting for resources, ready to be executed
Why might a process (or thread) be placed on the ready queue?
A process is placed on the ready queue when it is able to use a CPU. Some examples include:
- A process was blocked waiting a read from storage or socket to complete and data is now available.
- A new process has been created and is ready to start.
Other situations
- A process thread was blocked on a synchronization primitive (condition variable, semaphore,
mutexlock) but is now able to continue. - A process is blocked waiting for a system call to complete but a signal has been delivered and the signal handler needs to run.
Common Scheduling Algorithms
- Shortest Job First
- Priority queue
- First Come First Served
- Round Robin
- Quantum = 500ms (for example)
Which schedulers suffer from starvation?
- Example: Shortest Job First with continuous stream of short processes
Convoy Effect
Convoy of processes following a CPU-intensive process, with potentially smaller resource requirements.
Affects IO-intensive operations. FCFS suffers from this.
What is pre-emption?
When a more preferable (multiple criteria) process is ready, the CPU can suspend the current process (think SIGSTOP), and can switch in the new process. Later, the process that was pre-empted can be scheduled (SIGCONT)
Without pre-emption processes will run until they are unable to utilize the CPU any further!
Questions
Savvy Scheduler